Typically, steering racks for vehicle rack and pinion steering gears are manufactured from round solid bar stock, with the toothed region broached across the bar near one end. This results in the cross section of the toothed region having a ‘D’ shape and hence these racks are commonly referred to as “D-racks”. The toothed region of such a broached D-rack has significantly less bending strength than the round solid shank extending from it. However, to minimise the weight of the steering rack, it is desirable that the toothed region and the shank have similar bending strength. A common approach to this problem is to gun drill the shank over most of its length resulting in a substantially tubular shank. However, the disadvantages of gun drilling are that material is wasted and it is a relatively expensive process. A further disadvantage of broaching the toothed region of a rack is that broaching cannot produce variable ratio teeth.
An alternative method of manufacturing a steering rack from round solid bar stock is to forge the toothed region. U.S. Pat. Nos. 4,571,982 (Bishop) and 5,862,701 (Bishop et al) disclose die apparatus for flashless warm forging the toothed region to net shape. “Net shape” means that the forged rack teeth do not require any further machining after forging. An advantage of forging is that the rack teeth may be shaped to have a variable gear ratio. The cross section of the toothed region of racks forged by this type of die has a ‘Y’ shape and such racks are commonly referred to as “Y-racks”. The toothed region of a forged Y-rack has greater bending strength than the toothed region of a D-rack broached from the same diameter solid bar, and so Y-racks can be forged from smaller diameter bar whilst maintaining overall bending strength. However, the shanks of Y-racks are still commonly gun drilled to further reduce weight.
WO 2005/053875 A1 (Bishop Innovation Limited) discloses a die apparatus for flashless forging D-racks from a solid bar. Unlike a typical Y-rack, the cross sectional area of the toothed region of a typical solid D-rack is less than the cross sectional area of its shank prior to gun drilling. This means that a typical D-rack cannot be flashless forged from bar stock of constant diameter. This problem may be overcome by forging the D-rack from a solid bar having a smaller diameter in the region where the teeth are to be forged.
A prior art solution to the problem of gun drilling the shank of a solid rack is a “composite rack”, in which a solid toothed member is joined to a hollow shank by a joining method such as welding. Composite racks are potentially cost effective. However, there is some market resistance to a joint in a steering rack due to safety concerns.
Another alternative to gun drilling is disclosed in EP 0445470 B1 (Simon) in which a blank for a steering rack is made by extrusion from a short length of thick walled tube. The resulting hollow blank has a thick walled region on to which the teeth are cut and a region of thinner wall section for the shank. However, this method of a making a blank is not suitable for subsequent forging of the teeth, and hence not suitable for producing variable ratio teeth, because the hollow region where the teeth are to be formed is not suitable for use with the forging methods described above. DE 19901425 A1 (Umformtechnik Bauerle GmbH) discloses another method of making a hollow blank having a thickened region where the teeth are to be cut, but again it is not suitable for producing forged teeth.
Numerous attempts have been made to manufacture steering racks by forming tube stock. However, none of these methods have been widely used in mass production. A particular problem with manufacturing a steering rack directly from tube stock is that the teeth cannot be forged on to the tube unless the bore of the tube is supported against collapsing. Furthermore, the tube stock must have sufficient wall thickness to form teeth thereon, which may result in a tube thickness being chosen that is unnecessarily thick for the shank of the rack and therefore the rack as a whole is heavier than necessary. Forging methods have been proposed where a removable mandrel is placed in the bore of the tube stock during forging of the teeth. However, this method has a number of problems. The mandrel is difficult to remove after forging and the material must flow around the rigid mandrel.
Another method of forming a steering rack directly from tube involves firstly flattening one side of the tube then passing a series of mandrels through the flattened tube such that it is forced outwardly into a die to form the teeth. Such a method is disclosed in EP 0572105 B1 (Tube Forming Co Ltd). However, this method still suffers from the limitation that the tube must be of sufficient thickness to form the teeth. Furthermore, it is a relatively slow process and it is not well suited to producing variable ratio teeth that have large ratio changes.
A method of forging the teeth of a steering rack on to tube stock is disclosed in JP 57-195960A (Jidosha Kiki Co Ltd) in which a solid elongate member is positioned in the bore of a length of tube stock prior to forging. The solid elongate member supports the bore of the tube during forging and is retained in the tube as part of the finished steering rack. The method disclosed shows the teeth being forged on to tube stock having constant wall thickness. This means that the wall thickness must be sufficient to form the teeth on and therefore thicker than necessary for the shank of the rack, which results in a rack that is heavier than necessary. Furthermore, the solid elongate member cannot be relied upon to provide additional strength and support to the teeth forged on the tube. This is because the interface between the forged tube and the elongate member cannot transmit significant shear stress. The disclosure in this publication shows the teeth being forged on to a tube having a constant outside diameter, and as such it is not clear how the combined cross sectional area of the tube and the solid elongate member is reduced to that of the finished D-rack shown. A similar method is disclosed in JP 2000-016310 A (Kayaba Industry Co Ltd). In this disclosure, the elongate mandrel has a flat on one side to accommodate the reduction in cross sectional area required in the toothed region, but it still does not overcome the problem that the constant wall thickness tube stock shown in this disclosed method must have sufficient wall thickness to form teeth over its entire length.
The present invention seeks to provide a steering rack and a method of manufacture thereof that ameliorates at least some of the disadvantages of the prior art.